[VE] Implements minimum MC layer for VE (4/4)
This patch includes following items.
- Adds AsmParser and minimum AsmBackend/ELFObjectWriter/MCCodeEmitter to support only LEA instruction in order to reduce the size of this patch.
- Adds regression test of MC layer for a LEA instruction.
- Relocations are not supported this time to reduce the size of this patch.
Differential Revision: https://reviews.llvm.org/D79546